The invention relates to an ammonium phosphotellurate pyroelectric vidicon and detector. It applies more particularly therefore to the field of detecting thermal, e.g. infrared radiation and to the field of infrared and ultrasonic imaging. These methods use generally monocrystalline, dielectric materials having a spontaneous polarization, which is very temperature dependent. This property is more particularly encountered in ferroelectric dielectric materials.
In general, pyroelectric radiation detectors comprise a plate capacitor, as shown in FIG. 1, whose dielectric material 2 is generally a monodomain ferro-electric monocrystal, i.e. it is pyroelectric. The faces of the monocrystal in contact with electrodes 4 and 6 of the capacitor are perpendicular to the polar axis P of the monocrystal, i.e. the polar axis P is directed in accordance with the normal N to the electrodes.
The absorption of the incident radiation, particularly infrared radiation, on the surface or within the actual dielectric material, leads to a temperature rise dT of the material and causes a variation dP.sub.s in its spontaneous polarization P.sub.s. This leads to a voltage dV, which is applied to the input of a high impedance amplifier 8, e.g. constituted by a field effect transistor (FET).
More particularly in the case of infrared imaging, the upper electrode 5 of the capacitor is replaced, in the manner shown in FIG. 2, by a grid 10. The reading of the signal from the dielectric material 2 exposed to radiation is carried out by scanning point-by-point using an electron beam 12, the face of the device opposite that exposed by the radiation. This device, called a pyroelectric vidicon, makes it possible to restore on the actual device a video image of the illumination of the radiation, such as for example the image of an object from which the said radiation comes. As hereinbefore, the voltage dV which can differ at any point of the material resulting from the temperature rise of the latter, is applied to the input of a video amplifier 14.
In both these applications, the comparison and choice between the different pyroelectric materials takes place on the basis of qualitative and quantitative data.
From the quantitative standpoint, the most important parameters are as follows:
the pyroelectric coefficient p corresponding to the variation of the spontaneous polarization P.sub.s as a function of the temperature T, i.e. p=.delta.P.sub.s /.delta.T;
the principal dielectric permittivity .epsilon. along the polar axis;
the loss angle and its variation as a function of the frequency in the range between 1 and 100 Hz, measured along the polar axis;
the calorific capacity C;
the lateral thermal conductivity K, i.e. determined in the plane perpendicular to the polar axis.
There is no need to consider these various parameters in isolation, when comparing the different materials. In general, an overall figure of merit M defined by the ratio M=p/.epsilon.C is used.
From the qualitative standpoint, the most significant properties are:
the homogeneity of the aforementioned parameters and particularly the homogeneity of the pyroelectric response, which is defined by the ratio p/.epsilon. (particularly important in the case of the pyroelectric vidicon);
the ease of handling and conditioning, more particularly the ease of cutting and polishing very thin plates having a large surface;
the ease of obtaining large samples without any defects, with regards to the preparation of the material.
There are two main categories of ferroelectric materials which are conventionally used in pyroelectric detection. The first category consists of crystals which are insoluble in water, such as crystals of LiTaO.sub.3, Pb.sub.x Zr.sub.1-x TiO.sub.3, etc . . . , said materials also having high Curie temperatures. The second category consists of crystals having a Curie temperature close to ambient temperature and which are mainly soluble in water, such as triglycine sulphate (TGS), triglycine selenate (TGSe), triglycine fluoborate (TGFB), etc.
The first-mentioned materials have the advantage of being insensitive to moisture, but generally have a low pyroelectric response (low p/.epsilon.). The second-mentioned materials are sensitive to moisture and consequently more difficult to use. Thus, it is necessary to encapsulate them in a dry atmosphere or in vacuum.
When used in connection with imaging, the sensitivity to moisture is not a serious disadvantage, because the material must be placed in a vacuum compatible with reading by an electron beam. In this type of application the universally used material is TGS. Its figure of merit M, close to ambient temperature, is one of the highest known. Unfortunately, this compound has serious disadvantages.
Thus, it is very difficult to grow large, homogeneous, defect-free monocrystals. Only through making a very considerable effort in the crystallogenesis of the material is it possible to prepare samples usable in infrared imaging. Moreover, the cutting and polishing of very thin plates (approx. 30 .mu.m) is difficult to perform, due to the mediocre mechanical properties of this material.
With regards to the isomorphous materials of TGS, such as TGSe and TGFB, said materials have higher Curie temperatures than TGS and their figures of merit are also higher than those of TGS. Unfortunately, these materials are even more difficult to produce than TGS and their mechanical properties are not superior to those of the latter.